Biological reactors find increasing use in many areas of industry, including waste treatment plants. Efforts to protect the environment include advanced biological treatment of wastewater through the use of biological reactors, and in particular, fluidized-bed bioreactors. It is the activity of biologically active materials (or “biomass”) within the biological reactor that degrades contaminants in the influent to effect a filtration process. As the biomass treats these contaminants through enzymatic reaction, the biomass grows through reproduction within the system. Typically this activity occurs within a treatment vessel which contains media or other substrate material or carriers on which the biomass attaches and grows as contaminants are consumed. Typical media would include plastic beads, resin beads, sand, or ion exchange resins, among other carriers.
Conventional fluidized-bed bioreactors, such as well-mixed suspended carrier reactors (SCRs), suffer from operational limitations in that the media or carriers of the fluidized bed may be subject to excessive buildup of biomass and precipitates, thereby causing compromised flow distribution, excessive media and/or biomass carryover, crusting, increased clogging of filters, and the like. If not properly limited, biomass and precipitate buildup is detrimental to system performance. Uncontrolled biomass film growth in a fluidized bed biological reactor can also result in an undesirable loss of media.
Media bed expansion can, under certain circumstances, be limited by the application of shear, but the success of such a control strategy depends upon whether excess biomass and suspended solids can be transported to the top of the fluidized bed. More specifically, it is recognized that such transportation of excess biomass and suspended solids toward the top of the bed is promoted by several dominant mechanisms. For example, media grains that are coated with thicker layers of biomass tend to have an overall particle density that is less than the average particle density within the fluidized bed. Those particles, therefore, are transported to the top of the fluidized bed by virtue of upward moving fluid flow as well as the reduced particle density. This upward movement results in some shear forces acting on biomass-covered particles which does separate some biomass from its supportive media.
One solution to this problem has been to increase the amount and size of cavities introduced into the system to increase the shear and subsequent separation of the biomass from its media. An example of a bioreactor in which such a system is operated is shown in FIG. 1.
FIG. 1 illustrates vessel 100 which contains an aqueous suspension of biomass and media, such as would be used in a waste-water treatment plant. Vessel 100 is fed by inlet pipe 105 with a waste-water stream. The biomass covered media 101 is shown as relatively small dots, and air introduced into the system is shown as cavities 110. (Some would say bubbles. For purposes of this disclosure, however, it is intended that bubble and cavity be used interchangeably). In FIG. 1, cavities 110 are shown as roughly spherical cavities of gas which travel upward through the liquid contained in vessel 100. Typically the gas is air, although it could be a gas having an oxygen enriched content (as compared to air) or even be pure oxygen. Air sparger 120 is shown at the bottom of vessel 100 and is fed air (in a typical embodiment) from line 125. Air sparger 120 in this conventional embodiment would be a coarse diffuser to create relatively large cavities and thus increase the shear forces acting on the biomass covered media. These cavities of air thus produced travel upward through the liquid in vessel 100.
The introduction of air into the liquid via air sparger (air inlet) 120 serves two purposes. First, it supplies oxygen which is needed for the enzymatic reactions which are taking place in the system as contaminants are removed and biomass is formed on the media. Secondly, the upward cavity movement causes currents to be developed in the liquid. These currents will cause movement of the media and the application of shear stresses to the media and biomass. This interaction between pieces of media with other pieces of media, fluid, biomass, and the interior wall(s) of vessel 100 results in collisions which cause: (1) contact between the microorganisms and suspended organic matter, and (2) accumulated biomass to break free from its respective supportive media. This freed biomass will typically rise to the top of the vessel as its density is less than that of the liquid system. FIG. 1 shows a filter 140 which is used to filter out the biomass as “clean” effluent water is drawn from the system through outlet pipe 150.
Many of these types of systems rely on the filtering of treated effluent (e.g. purified water in the case of waste water treatment) at filter 140 to remove clean product water or to separate the bioreactor media from the effluent. This leads to shut-down time and other expensive aspects to the operation of the reactor. As product water is drawn off and fresh waste feed is caused to flow into the reactor, it becomes necessary to remove resultant biomass. With too great a biomass concentration in the reactor, filter fouling, plugging, excessive biomass in the effluent and other problems compound to affect the reactor performance.
Thus, there remains a need in the industry for an improved system for removing accumulated biomass from a slurry of a fluidized-bed bioreactor to inhibit uncontrolled biomass growth and precipitate accumulation. Objects and advantages of the invention will become apparent to those skilled in the art from the drawings, the detailed description of preferred embodiments, and the appended claims.